An organic light emitting device typically includes two electrodes, and an organic material layer interposed between the anode and the cathode, in which electric current is converted into visible light by injecting electrons and holes to the organic material layer, thereby emitting light.
FIG. 1 is a schematic diagram which illustrates a cross-sectional diagram of a general organic light emitting device as one of the conventional organic light emitting devices.
With reference to FIG. 1, the organic light emitting device comprises a light emitting layer (50) interposed between two electrodes, for example, an anode electrode (20), and a cathode electrode (70). Any one of the two electrodes, for example, the anode electrode (20) is disposed on a transparent substrate (10), and transmits light emitted from the light emitting layer (50). The organic light emitting device can further comprise at least one layer selected from the group consisting of an electron injecting layer (65), an electron transporting layer (60), a hole transporting layer (45), and a hole injecting layer (40) in order to improve the performance. Further, the organic light emitting device can comprise an insulating layer (30) on the electrode of the transparent substrate (10) in order to distinguish the light emitting region (A) from the non-light emitting region (B).
The two electrodes (20, 70) can be formed of a metal, a metal oxide or a conductive polymer. The materials for forming the electrodes can have unstable characteristics on the interface with the organic material layer. Further, heat applied from the outside, internal heat which can be generated upon driving the organic light emitting device, and the electrical field applied to the organic light emitting device can give an adverse effect on the performance of the device. The drive voltage can be increased for the device operation due to the difference in the conductive energy level between the electron/hole injecting layer (65/40) or the electron/hole transporting layer (60/45), and the organic material layer.
Accordingly, it is important to minimize the energy barrier to electron/hole injection/extraction from or to the electrode, and it is also important to stabilize the interface between the electron/hole injecting layer or electron/hole transporting layer, and the organic material layer, and thus a technique for improving this can be developed.
For this, for the anode electrode of the organic light emitting device, a material having HOMO (highest occupied molecular orbital) energy level such that the anode electrode is modulated to have a Fermi energy level similar to the HOMO energy level of the hole injecting layer, or having a HOMO energy level similar to the Fermi energy level of the anode electrode is selected as a hole injecting layer.
Since the hole injecting layer should be selected, taking into consideration not only the Fermi energy level of the anode electrode, but also the HOMO energy level of the hole transporting layer or the light emitting layer, there is some limitation on selection of the materials for the hole injecting layer. Accordingly, a method for modulating the Fermi energy of the anode electrode is generally employed for the preparation of the organic light emitting device.